The advection-dominated accretion flow for the anti-correlation between the X-ray photon index and the X-ray luminosity in neutron star low-mass X-ray binaries

Abstract

Observationally, an anti-correlation between the X-ray photon index $\Gamma$ (obtained by fitting the X-ray spectrum between 0.5 and 10 keV with a single power law) and the X-ray luminosity $L_{0.5-10keV}$, is found in neutron star low-mass X-ray binaries (NS-LMXBs) in the range of $L_{0.5-10keV}\sim 10^{34}-10^{36}\mathrm{erg}{\mathrm{s}}^{-1}$. In this paper, we explain the observed anti-correlation between $\Gamma$ and $L_{0.5-10keV}$ within the framework of the self-similar solution of the advection-dominated accretion flow (ADAF) around a weakly magnetized NS. The ADAF model intrinsically predicts an anti-correlation between $\Gamma$ and $L_{0.5-10keV}$. In the ADAF model, there is a key parameter, $f_{\mathrm{th}}$, which describes the fraction of the ADAF energy released at the surface of the NS as thermal emission to be scattered in the ADAF. We test the effect of $f_{\mathrm{th}}$ on the anti-correlation between $\Gamma$ and $L_{0.5-10keV}$. It is found that the value of $f_{\mathrm{th}}$ can significantly affect the anti-correlation between $\Gamma$ and $L_{0.5-10keV}$. Specifically, the anti-correlation between $\Gamma$ and $L_{0.5-10keV}$ becomes flatter with decreasing $f_{\mathrm{th}}$ as taking $f_{\mathrm{th}}=0.1,0.03,0.01,0.005$, $0.003$ and $0$ respectively. By comparing with a sample of non-pulsating NS-LMXBs with well measured $\Gamma$ and $L_{0.5-10keV}$, we find that indeed only a small value of $0.003\lesssim f_{\mathrm{th}}\lesssim 0.1$ is needed to match the observed anti-correlation between $\Gamma$ and $L_{0.5-10keV}$. Finally, we argue that the small value of $f_{\mathrm{th}}\lesssim 0.1$ derived in this paper further confirms our previous conclusion that the radiative efficiency of NSs with an ADAF accretion may not be as high as $\epsilon \sim \frac{\dot{M}GM}{R_{\ast }}/\dot{M}{c}^2\sim 0.2$.

arXiv comment12 pages, 12 figures. Accepted for publication by MNRAS

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